Lubricants are often used in demanding environments where degradation can be accelerated by high temperatures, extreme wear conditions, acidic or other corrosive conditions, etc. For example, the conditions under which automobile engines function are severe enough to require periodic oil changes to replace degraded engine lubricant in order to protect the engine against wear and damage that can lead to catastrophic failure.
Alkylated diaryl amines, such as alkylated diphenylamines (ADPAs), are well known antioxidants widely used to prevent degradation and maintain the performance of engine oils found in gasoline and diesel engines for cars and trucks, as well as a variety of industrial lubricants and lubricants for marine engines, etc. When selecting a diaryl amine antioxidant a number of performance, safety and environmental concerns must be addressed. For example, diphenylamine itself has good antioxidant activity but is known to be a sensitizer and its presence is typically kept to a minimum, e.g., less than 1% of any ADPA antioxidant. Diphenylamines substituted with hydrocarbyl groups are more soluble in lubricating oil and the higher molecular weight reduces volatility. Increased alkylation also helps to solubilize polar materials formed from oligomerization of spent oxidized amines, which reduces deposits, sludge and varnish. On the other hand, the antioxidant activity of ADPAs is dependent on the concentration of nitrogen provided and is thus inversely proportional to molecular weight and so excessive alkylation or very large alkyl groups should be avoided. NAUGALUBE 438L, a mixture of diphenylamines alkylated by one or more nonyl-chains derived from propylene trimer is an effective and widely used liquid antioxidant
Diaryl amines useful as anti-oxidants bearing substituents other than alkyl groups are known but such compounds are not as common in engine oils as alkyl substituted diaryl amines. For example, U.S. Pat. No. 7,704,931 includes 3-hydroxydiphenylamine and 4-hydroxydipenylamine in lists of possible antioxidants in a lubricant composition; U.S. Pat. No. 8,202,829 includes 3-hydroxydiphenylamine in a list of suitable antioxidants for use in a non-synthetic lubricating oil comprising less than 30 wt % monocycloparaffins and from 0.8 to 2.0 wt % tetracycloparaffins; and U.S. Pat. No. 7,569,526 includes 3-hydroxydiphenylamine and 4-hydroxydipenylamine in lists of possible antioxidants for use in the oil portion of a metal working fluid, but none of these three disclosures exemplify the use of a hydroxydiphenylamine.
U.S. Pat. No. 7,498,467 disclose aminophenol and hydroxydiphenylamine antioxidants wherein, on at least one phenyl ring, a hydroxyl substituent is adjacent to an amino substituent.
JP 2011-256314 discloses a composition comprising an aliphatic alkyl ester biodiesel fuel, which fuel may also contain a fossil fuel component, and an antioxidant of the formula

wherein n and M can be 0, 1 or 2 provided that m+n=1 or 2, and R is a C1-18 alkyl, which alkyl may be further substituted. Of the possible hydroxydiphenyl amine compounds of the above formula, only 4-hydroxydiphenylamine is exemplified.
GB 1,145,189 discloses the use of substituted 2-hydroxydiphenylamines as antioxidants in hydrocarbon and ester based lubricating oils.
EP 016559 discloses 3-hydroxy-4-styryldiphenylamine, which may also be further substituted by styryl at the 2- or 4′-positions, as an antioxidant for hydrocarbon and ester based lubricating oils. Compositions comprising ester based oils are exemplified.
“Thermoanalytic study of inhibitors of oxidation of synthetic oils” Kyazim-zade, A. K.; Gadirov, A. A.; Akchurina, T. Kh., Neftekhimiya (1996), 36(1), 73-75 investigated the thermal stability and the effect on the oxidation of pentaerythritol esters at elevated temperatures of certain 3-hydroxyl or 3-alkoxy-4-hexyldiarylamines of the following formula:

wherein R is hydrogen, butyl or hexyl and R′ is hydrogen or methyl.
Degradation of lubricating oil, such as oil in engine lubricants, can cause many undesirable effects, such as the formation of deposits, changes in viscosity and lubrictity, etc. A number of tests are commonly used in the industry to evaluate the effectiveness of antioxidants in lubricant compositions, e.g., TEOST measures deposits, PDSC is used to measure the onset of appreciable oxidation chemistry, etc.
It has been found that lubricating oil compositions comprising mixtures of an alkylated diarylamine, such as the commercially available nonyl-substituted diphenylamine NAUGALUBE 438L, with certain alkylated hydroxydiarylamines, i.e., alkylated diarylamines substituted by hydroxyl on one or more aromatic ring carbon atoms, exhibit excellent antioxidant and/or deposit control activity that in many cases is superior to the performance the alkylated diarylamine or alkylated hydroxydiarylamine alone.